A variable compression ratio mechanism capable of changing the compression ratio by using a piston-crank mechanism has been proposed such as disclosed in U.S. Patent Application Publication, No. 2004/0163614 A1. Such a variable compression ratio mechanism is constructed to control the compression ratio of the engine by changing the rotation position of a first control shaft by an actuator such as a motor in accordance with the engine operating condition.
In a structure in which the actuator for the variable compression ratio is placed outside the engine body to protect against oil, exhaust heat or the like, for example, an actuator and a first control shaft are coupled by a coupling mechanism while a first arm portion of the first control shaft disposed inside of the engine body and a second arm of a second control shaft disposed outside of the engine body are coupled by a lever extending through a side wall of the engine body. A second control shaft is accommodated in and attached to a housing attached to the side wall of the engine body, and an actuator such as an electric motor or the like is attached to the housing.
In the variable compression ratio internal combustion engine of such structure, by changing the rotation angle of the first control shaft, the compression ratio of engine is changed while, because of the change in posture of the first and second arm portions and a lever coupling the first arm portion and the second arm portion, the speed reduction ratio through a rotation power transmission path or line from the actuator to the first control shaft i.e. total reduction ratio is also changed.